This invention relates generally co power control within mobile communication systems and, in particular, to a system and method of power control within a Direct Sequence (DS)-Code Division Multiple Access (CDMA) system.
In DS-CDMA communication systems, efficient power control of mobile station (MS) to basestation (BS) communications, hereinafter referred to as reverse link communications, is essential. One key consideration with respect to the transmit power levels in reverse link communications is interference caused by other MSs transmitting on the same channel, hereinafter referred to as co-channel MSs, in the same or neighboring cells. Hence, reduced transmit power levels result in a reduction in co-channel interference problems. Another key consideration is the limited battery power within the MSs, making it important to minimize unnecessary power consumption at the MSs. On the other hand, reverse link communication channels have a particular Quality of Service (QoS) requirement that must be met with a minimum transmit power. Thus, the transmit power level at the MS is a balance between co-channel interference/battery utilization and QoS requirements.
If each radio channel is static and the transmit power of each MS is properly allocated based on its geographical location, the level of co-channel interference and battery power utilization can be minimized so as to not severely affect the BS receiver""s performance for any particular MS. However, since radio channels are time varying in a realistic environment, the signals sent from MSs and received at the BS experience power variations, due to the phenomenon known as channel fading, hereinafter referred to simply as fading.
To adjust for fading, previous standards for DS-CDMA communication systems have established the use of a power control command, consisting of a single power control bit, to be periodically transmitted from the BS to each MS communicating with the BS. Communications from the DS to the MS, hereinafter referred to as forward link communications, are used to transmit these power control commands. Each power control bit, according to previous setups, is generated in response to a power detection process for data signals received at the BS. These power control bits are systematically transmitted to the MSs in order to indicate whether the transmit power at a particular MS should be adjusted to compensate for power variations, such as fading, at the BS""s receiver. If the power associated with the received data signals is found to be below a predetermined acceptable power threshold level, the BS indicates this by transmitting a high value on the power control bit. On the other hand, if the power associated with the received data signals is above the threshold level, the DS indicates this by transmitting a low value. In response to these power control bits, the MS increases the transmit power if a high value was sent and decreases the transmit power if a low value was sent. This allows for a dynamically controlled transmit power at the MSs that can compensate for each MS""s particular situation.
The third generation DS-CDMA standard, referred to as the 3G DS-CDMA standard or cdma2000 in North America has been defined with increased flexibility when compared to previous standards. In the reverse link of a cdma2000 system, an MS is capable of simultaneously transmitting one pilot channel and one data channel, the data channel capable of being formed by code-multiplexing multiple code-subchannels that include fundamental, supplemental, and dedicated control channels. The pilot channel and the code-subchannels all pass through the same radio transmission environment and thus, all the signals experience the same fading. Because of this fact and the fact that code-subchannel signals are designed to be mutually orthogonal, it has been assumed that it is appropriate to apply a single bit to power control the entire MS transmitter, and this is consistent with proposed power control systems for cdma2000.
A problem with this approach is that when the subchannels are demodulated, the residual interference seen by each code-subchannel signal may be different clue to loss of orthogonality caused by the existence of multipaths in the radio transmission environment, and the different processing gains of the code-multiplexed subchannels. Thus, the variation of each code-subchannel""s signal-to-interference-plus-noise ratio (SINR) may be different for a given fading rate. Further, each of these code-subchannels has its own quality of service (QoS) requirement. Hence, different code-subchannels can have different power limitations and requirements at the BS""s receiver. The simple conventional power control systems discussed above are incapable of addressing these issues and may require more transmit power from the MSs than what would be ideally required to achieve the same QoSs.
It is an object of the present invention to overcome at least one of the disadvantages of the prior art and, in particular, to provide for more efficient selection of transmit power levels at MSs within mobile communication systems.
The invention provides an improved power control method for controlling a mobile station which transmits using several physical channels. A base station makes measurements for each of the physical channels independently, and as a function of those, produces either a single flower control bit which is used at the mobile station to adjust all transmit gains, or a power control bit for each physical channel which the mobile station uses to adjust transmit gains of each physical channel independently.
The present invention is, according to a first broad aspect, a method of controlling the transmit power of a mobile station that is operable to transmit a plurality of physical channels to a basestation simultaneously. The method includes steps of making power characteristic estimates for at least two of the physical channels and controlling the transmit power at the mobile station with use of the power characteristic estimates.
The present invention, according to a second broad aspect, is a power control apparatus arranged to be implemented within a basestation that can receive a plurality of physical channels from a mobile station simultaneously. In this aspect, the power control apparatus includes a plurality of receivers, a plurality of estimation devices, a power control bit generator and a power control bit transmitter. Each of the receivers preferably receives a data signal from the mobile station and separates a respective physical channel within the data signal. Each estimation device preferably receives one of the individual physical channels and makes a power characteristic estimate for the received physical channel. The power control bit generator preferably receives the power characteristic estimates and generates at least one power control bit on the basis of the estimates. The power control bit transmitter preferably transmits the power control bit to the mobile station.
The present invention, according to a third broad aspect, is a basestation including a receiver, a power control apparatus, and a transmitter. The receiver operates to receive a plurality of physical channels from a mobile station simultaneously, The power control apparatus operates to make power characteristic estimates for at least two of said physical channels. The transmitter operates to send transmit power control information to the mobile station with use of the power characteristic estimates.
The present invention, according to a fourth broad aspect, is a power control apparatus arranged to be implemented within a mobile station that is operable to transmit a plurality of physical channels to a basestation. The power control apparatus receives at least two power control bits from the basestation that correspond to different physical channels and adjusts transmit powers corresponding to the physical channels on the basis of the received power control bits.
In further aspects, the present invention is a basestation that operates to generate a plurality of power control bits and transfer them to a mobile station, and a mobile station that operates to receive a plurality of power control bits and adjust the transmit power for a plurality of physical channels on the basis of the received power control bits.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.